1. Field of Endeavor
The present invention relates to reservoir interrogation and more particularly to encapsulated tracers for reservoir interrogation.
2. State of Technology
U.S. Pat. No. 4,555,488 for tracer chemicals for a method for determining flow patterns in subterranean petroleum and mineral containing formations using organonitrogen tracers provides the state of technology information reproduced below. The disclosure of U.S. Pat. No. 4,555,488 is incorporated herein in its entirety for all purposes.
In recovery of petroleum or minerals from subterranean formations, especially by chemical flooding, it is desirable to know the flow patterns of the formation prior to injection of chemicals. Tracers are used in such reservoir engineering. In the actual recovery process, during chemical injections, it is advantageous to follow the flow of each slug by using a tracer in the slug. Ideally, a tracer should be water soluble and inert to the solids and liquids in the formation. By inert is meant that it does not get absorbed onto the rocks; it does not partition into any oil phase which may be present; and it does not interact with the organics and minerals present in the formations. When injected with another chemical agent, it should also be inert to the chemical it is injected with. A tracer should also be easily and accurately detected by simple methods without interference by any substances present in the connate fluids.
The tracers now employed are radioactive isotopes and compounds like potassium iodide, ammonium thiocyanate and dichromate. Radioactive isotopes are expensive and require special handling by licensed personnel. Potassium iodide and alike are detected by wet analyses and, therefore, bear the limitations of such analyses.
U.S. Pat. No. 5,246,860 for tracer chemicals for use in monitoring subterranean fluids provides the state of technology information reproduced below. The disclosure of U.S. Pat. No. 5,246,860 is incorporated herein in its entirety for all purposes.
In most production reservoirs, the produced brines are injected into the formation for purposes of maintaining reservoir pressure and avoiding subsidence and environmental pollution. In the case of geothermal fields, the brines are also injected to recharge the formation. However, the injected brines can adversely affect the fluids produced from the reservoir. For example, in geothermal fields, the injected brine can lower the temperature of the produced fluids by mixing with the hotter formation fluids. In order to mitigate this problem, the subsurface paths of the injected fluids must be known.
Tracers have been used to label fluids in order to track fluid movement and monitor chemical changes of the injected fluid. Despite their importance to the reservoir operator, very few tracers are presently available. Furthermore, of those that are available, little is known about their stabilities or behavior at the elevated temperatures that typify geothermal resources capable of electric power generation.
Radioactive materials are one class of commonly used tracers. These tracers have several drawbacks. One drawback is that they require special handling because of the danger posed to personnel and the environment. Another drawback is the alteration by the radioactive materials of the natural isotope ratio indigenous to the reservoir—thereby interfering with scientific analysis of the reservoir fluid characteristics. In addition, the half life of radioactive tracers tends to be either too long or too short for practical use.
United States Patent Application 2010/0307745 for the use of encapsulated tracers provides the state of technology information reproduced below. The disclosure of United States Patent Application 2010/0307745 is incorporated herein in its entirety for all purposes.
The use of tracers to obtain information about an oil reservoir and/or about what is taking place therein has been practiced for several decades and has been described in numerous documents. Primarily tracers have been used to monitor fluid paths and velocities. More than one tracer substance may be used concurrently. For instance U.S. Pat. No. 5,892,147 discloses a procedure in which pluralities of different tracer substances are placed at respective locations along the length of a well penetrating a reservoir. The tracer substances are placed at these locations during completion of the well before production begins. The tracer at each location is either attached to a section of pipe before it is placed at that location or is delivered into the location whilst perforating casing at that location. When production begins, monitoring the proportions of the individual tracers in the oil or gas produced by the well permits calculation of the proportions of oil or gas being produced from different zones of the reservoir.
Distinctive chemicals which can be detected in high dilution, such as fluorocarbons, dyes or fluorescers have been used as tracers. Genetically coded material has been proposed (and WO2007/132137 gives a method for detection of biological tags). Radio-isotopes have frequently been used as tracers. Society of Petroleum Engineers paper SPE109,969 discloses the use of materials which can be activated to become short lived radio-isotopes.
Hydraulic fracturing is a well-established technique for stimulating production from a hydrocarbon reservoir. Typically a thickened, viscous fracturing fluid is pumped into the reservoir formation through a wellbore and fractures the formation. Thickened fluid is then also used to carry a particulate proppant into the fracture. The fracturing fluid is subsequently pumped out and hydrocarbon production is resumed. As the fracturing fluid encounters the porous reservoir formation a filtercake of solids from the fracturing fluid builds up on the surface of the rock constituting the formation. After fracturing has taken place a breaker (which is usually an oxidizing agent, an acid or an enzyme) may be introduced to break down this filter cake and/or to reduce the viscosity of the fluid in the fracture and allow it to be pumped out more effectively.
Tracers have been used in connection with hydraulic fracturing, mainly to provide information on the location and orientation of the fracture, as for instance in SPE 36675 and U.S. Pat. No. 3,987,850. U.S. Pat. No. 3,796,883 describes a further use of radio-active tracers to monitor the functioning of a well gravel pack.
It is known to associate tracers with a carrier material as particles from which the tracer is released after those particles are placed within a subterranean reservoir. For instance U.S. Pat. No. 6,723,683 uses starch particles as a carrier for a variety of oilfield chemicals including tracers. Association of a tracer substance with a carrier is also disclosed in U.S. Pat. No. 7,032,662 and U.S. Pat. No. 7,347,260.
U.S. Pat. No. 6,645,769 proposes that multiple tracers should be located at respective zones of a reservoir during completion of a well and also proposes that individual tracers should be associated with carrier particles from which the tracers are eventually released into the reservoir and hence into fluid produced from the well. This document teaches that placing of tracers at an individual location during completion of the well may be achieved by immobilization on a filter or casing before that filter or section of casing is inserted into the well.
United States Patent Application 2010/0307744 for the use of encapsulated chemical during fracturing provides the state of technology information reproduced below. The disclosure of United States Patent Application 2010/0307744 is incorporated herein in its entirety for all purposes.
It is well-known to deliver so-called oilfield chemicals (using this common term to include chemicals used in connection with either natural gas or oil and to include biochemicals such as nucleic acids and enzymes) to a subterranean hydrocarbon reservoir to bring about a variety of functions at various stages of hydrocarbon production. Methods for delivering oilfield chemicals to a reservoir include methods in which the chemical is made into the form of particles which are suspended in the fluid which is pumped down a wellbore to the reservoir. Common methods for forming particles are absorption into the pores of porous carrier particles and encapsulation as a core-shell structure in which a single quantity (the core) of the oilfield chemical is enclosed within a shell of carrier material.
Hydraulic fracturing is a well-established technique for stimulating production from a hydrocarbon reservoir. In a conventional fracturing procedure a thickened aqueous fracturing fluid is pumped into the reservoir formation through a wellbore and opens a fracture in the formation. Thickened fluid is then also used to carry a particulate proppant into the fracture. Once the fracture has been made and packed with proppant, pumping is stopped. The formation closes onto the proppant pack and oil or gas can flow through the proppant pack to the wellbore. At least some of the aqueous fracturing fluid in the wellbore will be driven back to the surface by fluid produced from the reservoir. Thickener which increases the viscosity of the fracturing fluid may be a polysaccharide. Guar gum, often crosslinked with borate or a zirconium compound is frequently used. Another category of thickeners which is used is viscoelastic surfactants. An oilfield chemical may be delivered to a reservoir during fracturing. If the fracturing fluid contains a viscosifying thickener, it is normal to supply a so-called breaker (which is usually a chemical or an enzyme) into the fracture to degrade the thickener and so reduce the viscosity of the fluid in the fracture after it has served its purpose. This facilitates the flow back to the surface and the flow of produced fluid through the proppant pack towards the wellbore.
U.S. Pat. No. 4,506,734 teaches the encapsulation of a breaker chemical, which may be an enzyme, within particles which are crushed by the fracture when pumping ceases and the fracture is allowed to close onto the proppant pack. This of course delays release of the breaker chemical until the fracture formation is complete.
U.S. Pat. No. 5,437,331 discloses an encapsulated enzyme breaker formed by a procedure in which porous beads of polymer are made and exposed to a solution of the enzyme which is absorbed into the open pores of the polymer beads. Examples in the document showed that this delayed release of the enzyme compared to incorporating enzyme solution directly into a model fracturing fluid.
Proposals for encapsulation of oilfield chemicals in contexts other than hydraulic fracturing include U.S. Pat. No. 6,818,594 which teaches the use of enzymes which are enclosed within a polymer capsule as a breaker for filtercake formed while drilling a well.
International Publication No. WO 03/106809 teaches particles in which an oilfield chemical (in the form of small droplets of aqueous solution) is enclosed in a matrix of an encapsulating polymer. This polymer is chosen so as to be soluble or otherwise degradable under conditions which are encountered within the reservoir after mixing with formation fluid found in the reservoir. The document teaches that these encapsulated particles should be made so small (mean particle diameter below 10 micron) that they can enter the pores of formation rock. Although delivery of particles to a reservoir via a production well is mentioned, an alternative possibility which is suggested is that particles can be delivered to the reservoir via an injection well and then flow through the formation to the vicinity of a production well to release the encapsulated chemical (a scale inhibitor) in the near wellbore region of the production well. This indicates that release of the encapsulated chemical will be sufficiently slow to allow time for travel through the formation from the injection well to the production well.
U.S. Pat. Nos. 4,506,734, 5,437,331, 6,818,594, 7,032,662, 7,347,260 and International Publication No. WO 03/106809 describes the various apparatuses, methods, and systems for encapsulating materials for use in subterranean formations. The disclosures of the above noted U.S. Pat. Nos. 4,506,734, 5,437,331, 6,818,594, 7,032,662, 7,347,260 and International Publication No. WO 03/106809, are incorporated herein in their entirety for all purposes.